The present invention relates to a device which makes it possible to detect the presence and the position of a ream of paper sheets, in particular sheets of securities or banknotes, on pallets. The present invention also relates to a method for such a detecting operation.
A problem which is frequently encountered in the production of printed sheets is the precise positioning of the sheet stacks on the pallets. This is because, in the case of a process for printing paper sheets, for example for securities or banknotes, sheet stacks are often transported on pallets from one machine to the other in order to run through the various steps of the process. It is thus the case that a sheet stack is fed to a given machine on a pallet and transferred into the inlet of the machine, the sheets ale introduced individually into the machine, in order for it to be possible for the machine to carry out its process step, for example printing, numbering or the like, and the sheet stack is reinstated on a pallet at the outlet of the machine.
In the case of an industrial production process, it is, of course, the case that, rather than just a single pallet being fed to the inlet of a machine, a continuous flow of pallets and sheet stacks is guided to and from each machine.
While the formation of sheet stacks at the outlet of a machine is straightforward, each sheet which has been processed by the machine passing out individually, it is more complicated for sheet stacks to be transferred from pallets to the inlet of the machine.
The sheets stacked up in a pallet have to be reliably transferred from the pallet to the feed table of the machine. This transfer operation call take place in various various for example, the pallet can be arranged on a feed table, on which the pallet is located in a known reference position of a transfer system; the transfer system, which comprises in particular a suitable gripper system for gripping the sheet stack, can be fed; a crosspiece can be lowered, in order for the pallet to be connected to the feed table of the machine, and the stack can be conveyed from the pallet to the feed table.
At this stage, it has appeared necessary to carry out sensing of the presence and of the position of the sheet stack on the pallet. This is because, if there is no sheet located on the pallet, it is necessary to avoid the above-described transfer process and to change the pallet instead and, if the sheet stack is positioned incorrectly on the pallet, for example if it is located too close to the border of the pallet on which the crosspiece is supported or if it is oriented obliquely, it may obstruct the movement of the crosspiece, which will then result in machine blockage and renders the intervention of an operator necessary. In the case of an industrial production process, such time losses are not acceptable and have to be reduced to a minimum.
Nevertheless, precise and reliable detection of the presence and/or of the position of a sheet stack on a pallet raises numerous problems.
A first problem is that it may be the case that the sheet stacks are supplied on pallets made of different material. It is thus possible for stacked sheets to be received on pallets made of metal, for example of steel, which have a reflective surface, or on pallets made of plastic, which have a matt surface. The sensing system thus has to be capable of sensing the presence of sheets or sheet stacks irrespective of the type of pallet used, each type of pallet, for this reason, having a different reflection coefficient.
A second problem is that sheets printed in different colors may affect the performance of the sensing system used: since the different colors have a different reflection behavior, it is possible that the sensing system may not be able to sense the presence of sheets on a given pallet.
Finally, where securities or banknotes are concerned, holograms are often used as a security element. A hologram, however, likewise appears to affect the performance of the sensing system. On account of the particular reflection behavior of a hologram, it may be the case that the sensor used does not sense the presence of sheets, to be precise in particular if the sensing operation takes place at the location where the hologram is situated. It is difficult, or even impossible, for the sensor to be reliably adjusted for definite sensing.